Electroacoustic transducer



April 1, 1958 R. B. CRANDELL ETAL 2,829,361

ELECTROACOUSTIC TRANSDUCER Filed Oct. 1, 1945 SWEEP CIRCUITS Fig.5.

Inventors:

Robert B. Crandell; Henrg C. Maulshagen, Richard W. Samsel,

Thei'h Attorneg.

2,829,361 a p ntncrnozscousrrcTR NSDUCE Robert Bt Crandell, Henry C. Maulshagen, and Richard ";W.. Samsel, Schenectady, 'N. Y., assignors to General Electric Company, a corporation of New York 'Application October 1, 1945, Serial No. 619,726

1 8 Claims- 340-10) This invention maestro .electroacoustic transducers and particularly to such devices which include piezoelectriccrystals and areemployed for the transmission or reception-of underwater soundwaves.

.In apparatus for transmitting orreceiving sound underwaterLit is necessary to provide transducers which can be subrnerged in Wateror other liquids for long periods oftime without damage. or deterioration. .It is, therefore, necessary to provide some form of leakproof casing for theftransducer through which casing the sound waves may be transmitted with minimum attenuation. For" some transducers it is desirable to employ piezoelectric crystals such as Rochelle salt crystals which are soluble in water, and the casings for such transducers must prevent leakage in order to preserve the crystals. In some .underwatei-[sound systems high frequency or supersonic waves are employed. The crystals ofi receiving transducers operating at supersonic frequencies are constructedwith dimensions most favorable to thefrecepn'on of weaksignals, and the .higher the operating frequency the smaller must be the individual crystals in the receiving transducer. The size and shape of the crystal isselected so that the crystal will resonate at the selected frequency. In underwater supersonic apparatus such as that disclosed and claimed in, applicationSerial- No. 619,720, filed concurrently herewith in the name offCramer La Pierre et al., and "assigned to the sameassighee it is necessary to mount alarge number of'crystalsin each transducer, and in the receiving transducerthese crystalsare veryv small. Rochelle saltgcrystals'for example, are difficultto handle and when used in underwater devices are preferably surroundedjor immersed in a liquid, such as castor oil, ih'whichthey are not soluble. When the transducer ineludes a plurality of crystals employed to handle difiere nt signaIsQ'it is necessarytoainsulate each crystal against both electr'ical and mechanical interference. When such crystals are very small the: mounting of the crystals and the connectingof -the crystals to theelectrical circuits thereof become extremelydiflicult: Accordingly, it is an object of this invention to' provide in an electroacoustic transducer employing a plurality of piezoelectric crystals an improved arrangement for mounting the crystals. p p

It is another objecbof this invention to provide an electroacoustic transducerffor underwater sound apparatus employing "piezoelectric crystals and including an. improved arrangement for mounting the crystals and for sealing the transducer against leakage.

.The features of this invention which ardbelieved to i be novel areset forth with particularity inthe appended claims. The invention itself, however, both as to its organization and method ofoperation, together with further objects and advantages thereof, may best be understood'by reference to the following description taken inconnectionwith the accompanyingdrawing in whichFig. 1 is a diagrammatic representationfan Patented Apr. 1, 1958 Fig, 3 is an enlarged face view, partly in section, of the.

transducer employed in Fig. 1; Fig. 4 is a longitudinal sectional side elevation of the transducer shown in Fig. 3 taken on the line 4--4 of Fig. 5; Fig. 5 is a sectional elevation view on the line 55 of Fig. 4; and Fig. 6- isv a diagrammatic view of two of the crystals and their electrodes as employed in the transducer of Figs. 3, 4

and 5.

Referring now to the drawing, Fig. 1 represents a supersonic wave receiving apparatus for the location of objects under water and which may form a part of a locating apparatus such as that described in the aforementioned La Pierre et 211.. application. This apparatus includes a signal receiving device 1 arranged to be mounted under water and to berotated to pick up signals over a predetermined field, and a signal portraying cathode raytube 2 on which the signals picked up by .the device 1 appear. The receiving apparatus 1 includes 'a parabolic reflector 3 arranged to concentrate the signals in predetermined positions along a line of piezoelectric crystals mounted in the receiving transducer 4. During the operation of the apparatus the crystals in the receiver 4 are scanned in order by a scanner ,5, and the signals are amplified by electronic amplifier 6 and are transmitted to the stationary equipment through suitable rotating connections (not shown). .The signals.

are then amplified by an intermediate frequency amplifier and detector .7 which impresses'the signals on 'a grid 8 of the cathode ray tube 2. The operation of the scan-.

ner also provides a synchronizing signal which is employed'to time the sawtooth wave sweep current generated by sweep circuits 9 and supplied to deflecting or sweep coils 10 mounted 'for rotation about the neck of the cathode ray tube 2. The receiving apparatus 1 1d, and as thelcoils rotate this radial movement of the beam is timed to occur each time the line of crystals in the transducer 4 isscanned The active or energized period of the .transduceris limited to a predetermined arc or'portion of the circular'field covered by the rotation of the receiving equipment 1. As shown in Fig.

2a portion ,of:.a sector of the fluorescent screen of the tube 2, represented bya dotted line 14, gives a visual presentation of the field .under examination and which.

is actively scannedduring each revolution of the re ceiver. The transducer, 4 is arranged so that when an object is present in the .water in fr ont of'the receiver parabola 3 it will result in the reception of. signals indicating the position andrelative size of the object as shown at 15. The purpose of theplurality of crystals employed in the transducer4 is to prolvide a plurality of pointsfalong each of the radii scanned by the apparatus as itreceives signals from the area 14. Each' scanning of the transducer 4 produces signals along a the fluorescent screen.

single radius of the segment 14," and because of the rotation of the receiver the series ofconsecutive scannings represent a series ofradii in the'seg'mental field under examination. If an object is present in the water in p the field under examination it is represented by brighter signals as indicated at 15, the signals appearing from the scanning of a plurality of adjacent radii combin'i'fi'g to produce an image orshadowgraph of the object on In order that the intensity of signals received along the line scanned by the apparatusmay be as great as possible, it is desirable that the crystals along the line be located at the focus points for the energy received from their respective different portions of the field. Objects directly in front of the reflector produce signals at the normal focus point for the parabola for beams parallel to the axis. The focus points for objects in the field either above or below the reflector lie along a curve which must be determined for the particular reflector under consideration and for sound at the frequency transmitted by the apparatus. The curve for the locus points for sound energy does not coincide with the curve for light, because the far greater wavelength of the sound energy must be taken into consideration. Furthermore the direction in which the flat faces of the crystals are pointed must also be taken into consideration, it being desirable to have the crystal point toward the center of that portion of the reflecor from which energy is to be received as determined by the direction from which it strikes the reflector. In the construction of a practical sound receiving equipment it normally will be found necessary to design the curvature of the line of crystals in accordance with good practice to provide a compromise between several factors involved. The degree of accuracy in the location of the crystals will, of course, vary with the degree ofprecision required in the operation of the equipment, greater accuracy being required for greater definition of the signal portrayed on the viewing screen. In some cases, for practical reasons it may be desirable to provide two or more lines of crystals in a single surface as in the construction illustrated; obviously the curve of the line of crystals employed in this case is a compromise since the two lines of crystals are operated at different frequencies although they lie along the same locus curve.

In order to secure reliable operation of the transducer 4, it is constructed so that highly sensitive crystals having dimensions of'the order necessary for resonance at the desired supersonic frequencies may be securely mounted with a minimum possibility of injury during operation. Referring. now to Figs. 3, 4 and 5, the transducer 4 comprises a metal casing or body 16, and a block 17 of resilient insulating material is mounted in the easing 16 against a partition member or inwardly extending flange 18. The block 17 may be constructed of any resilient material having the desired electrical and vibration insulating properties; for example, the material sold under the trade name Corprene has been found satisfactory for this purpose. The flange 18 divides the casing into two compartments or chambers in open communication with one another, the upper chamber containing the block 17 and the lower chamber indicated at 19 containing the outgoing connections to the several crystal's. The outer face of the block 17 indicated at 20, is constructed to include two inwardly sloping faces approximating a curve which would represent the locus of focus, points of the parabolic mirror 3 for the sound energy at the desired frequency. In the face 20 of the block 17'there are molded or otherwise suitably formed a plurality of recesses 21 arranged in a row along one side of the face of the block and a plurality of smaller recesses 22 arranged in a second row along the other side of the face. Slits 23 and 24 are formed along the outer sides of the rows of recesses, and conductor ribbons or electrodes 25 and 26, respectively, are pressed into the slits so that the conductors .25 and 26 enter each recess of the rows of recesses 21 and 22 respectively. On the opposite sides of the recesses there are inserted individual electrodes 27 and 28 respectively, and between the electrodes are. the piezoelectriccrystalsconstituting the signal sensitive: elements of the transducer. The larger crystals indicated at 29 are mounted in therecesses. 21 and the smaller crystals 30 in the recesses 22. The faces of the 4 crystals are flush with the face of the block 17. The dimensions of the crystals and the electrodes are such that the crystals are held in position in the recesses and between the electrodes by friction maintained because of the resilience of the material of the block 17. This arrangement for mounting crystals to be frictionally retained in a resilient block is described and claimed in a copending application of Richard W. Samsel, Serial No. 619,724, now abandoned, filed concurrently herewith and assigned to the same assignee. In order to insure suflicient and uniform pressure throughout the rows of crystals, the block 17 is constructed with an upper reduced portion in which the crystals are mounted, and metal clamping bars 31 are provided which may be drawn together against this portion of the block by screws 32. Each of the electrodes 25, 26, 27 and 28 is provided with conductors 33, 34, and 36, respectively, whereby the electrodes may be connected in the electrical circuits ofthe apparatus. These conductors extend from the electrodes through the resilient block 17 to the side opposite the face 20. The electrodes 25 and 26 which are common to all the crystals in their respective rows are grounded, the conductors 33 and 34 being connected to a common ground conductor 37 as indicated in Fig. 6. It is essential that each conductor be positioned so that it may be identified and properly connected in. the

circuit, and for this purpose spacing bars 38 and 39, and

a clamping bar are provided in the chamber 19 below the opening formed by the flange 18. As indicated in Fig. 5 the leads to the several conductors may be clamped between the bars 38, 39 and 40 and may then be connected to their respective ones of the conductors 33,. 34, 35 and'36- The conductor leads are preferably insulated cables having metallic sheaths, all the sheaths being grounded. when clamped between the bars 38, 39 and 40; this provides effective shielding for the several signal conductors and minimizes. cross talk. The chamber 19 is sealed by a plate 41 provided with a sealing gasket 42 and access to the chamber for adjustment of the spacing blocks and connections may be had by removing this plate. The connections to the several conductors are taken. out through a conduit connection 43 communicating with the chamber 19 at one side thereof; air or other gas under pressure may be admitted through the connection 43 to minimize the likelihood of leakage into the casin In iome types of apparatus it has been found desirable to employ Rochelle salt crystals; these crystals are soluble in water, and a protective casing is necessary to prevent water. from coming in contact with the crystals. Furthermore, when any crystals in a group operate on different.

signals, Water would short circuit the signal energy and cause cross talk between the several crystals; an insulating liquid is therefore required in such multiple signal transducers, whether or not the crystals are soluble in Water; In orderto prevent loss of signal strength the surrounding casing must besuch that there is little attenuation of the signal, due. to the passing of the sound through the casing Wall to the crystals. Rochelle salt crystals may be protected satisfactorily by immersion in suitable liquids, for example, castor oil; and, in the transducer illustrated, the crystals are maintained immersed in a. body of castor oil 44 which is contained within a casing formed by a rectangular cup or cover member 45. The cover is constructed of a flexible waterproof material, such as a suitable type of rubber, having sound transmission characteristics as nearly as possible the same. as those of the Water in which the apparatus is to be used. The cover 45 has a flange portion 46 and this portion, together with a metal collar 47, fits about the'bliock't17-andtis clamped to the edge of the casing 16v by a. plurality-ofscrews 48, a gasket 49 being located betweenthe collar; 47 and the case. The collar 47 also extends over the edge portion of the block 17 between out the operation the block and the clamps 31, andwhen the screws 48 aretightened the casing is not only sealed against leakage between the flange 46 of the cover and the case but, in addition, the collar 47 pressesthe block 17 agamst the flange .18 and efiectively prevents leakage of the castor oil '44 between the block and the casing. In the event that there is a tendency toward leakage of castor oil through the block 17 along the conductors, the lower s1de of the block within the flange 18 may be coated with a suitable leak-preventing cement; the face of the block and crystals may also be coated with a material to prevent leakage of the oil into the recess. .Smce the clamps 31 are metallic, they may tend to produce reflected signals with resulting distortion to the desired signals, and for minimizing such reflections an insulating collar or gasket 50 may be cemented or otherwise secured to the upper sides of the clamps 31. i b b The entire assembly as described is simple and compact and retains the crystals in their required positions for the reception of relatively weak signals with minimum distortion and interference. or cross talk between individualcircuits. The construction also facilitates the mounting of the crystals in the desired line representing the line of focus points for a reflector used therewith.

' Furthermore, although the crystals may be extremely less than a tenth of an inch, still the resiliency of the block 17 maintains the, crystals in position and in good electrical contact with their respective electrodes through- Fig. 1.

While the invention has been illustrated in connection with an electroacoustic transducer for receiving supersonic sound vibrations under water, other applications will readily be apparent to those skilled in the art; for example, the mounting of crystals in a resilient block in the manner disclosed may obviously be employed for the mounting of.crys'tals in transmitting transducers. It is not, therefore, desired that the invention be limited to the.

particular constructionillustrated and described, and it is intended by the'appended claims to cover allmodificaapproximating th line of focus points for signals reflected onto said transducer andsaid crystals having faces lying substantially along saidline, and electrode means engaging said crystals for from said crystals.

2. An electroacoustic transducer comprising a support, a block of resilient insulatingmaterial mounted on said support and having a row of recesses therein, said block conducting individual signals also having a slit providing communication between said recesses, piezoelectric crytals embedded in sa d recesses to form a line of crystals, a common strip electrode for said crystals arranged in said slit incontact with said crystals, individual electrodes in said recesses on the side opposite said slit each contacting a respective one of said crystals, and means for compressing said block to maintain said crystals and said electrodes in contact.

3. An electroacoustic [transducer comprising a case having a partition wallforming two chambers in open communication, a block of resilient insulating material in one of said chambers and closing the. opening between of a system such as that shown in spanner said chambers, said block having a recess therein on the outwardly facing" side thereof, 'a piezoelectric crystal 'in said recess, contact electrodes in said recess in engagement with said crystal, conductors for said electrodes" extending through said block and through the opening between said chambers into-the other of said chambers, a resilient closure niernber'for' said one chamber, a body of oil filling the space in said one chamber between said closure member and said block, and clamping means for pressing said closure member and said block into sealing engagement with said case to prevent the leakage of oil from said one chamber.

4. An electroacoustic transducer comprising an open case, a block of resilient insulating material in said case, a piezoelectric crystal mounted in said block, a wall member constituting a stop for determining the position of said block in said case, a resilient gasket mounted on the outer face of the Wall of said case about the opening therein, acollar resting against said gasket and against an adjacent face area of said block, a cover for said case constructed of resilient insulating material and having an edge portion resting against said collar, a second collar arranged over said edge portion, a body of insulating liquid filling said case, and means for clamping said second collar and said case together for compressing said gasket and the edge portion of said cover to prevent leakage of said insulating liquid and for pressing said first collar against said block to urge said block against said wall member.

5. An electroacousti-c transducer comprising a support, an elongated block of resilient insulating material mounted on said support, said block having a row of recesses there in opening on one face thereof, a plurality of piezoelectric crystals each arranged in a respective one of said recesses, contact electrodes for said crystals in said recesses laterally of said row, and clamping means arranged longitudinally of said block for compressing said block to retain said crystals in said recesses and in contact with said electrodes throughout the length of said row.

6. An electroacoustic transducer comprising a casing having two open sides and an inwardly extending flange near one of said open sides, a block of resilient insulating material resting against said flange and closing said one side of said casing, a plurality of piezoelectric crystals embedded in an outer face of said block, individual conductors for connecting said crystals in an electrical circuit, said conductors extending through said block into said casing, a spacing rack for said conductors within said casing, a conduit connection for leading said conductors from said casing, and a removable cover for the other of said sides for sealing said casing and for affording access to said rack.

7. An electroacoustic transducer comprising a casing having two open sides and an inwardly extending flange near one of said open sides, a block of resilient insulating material resting against said flange and closing said one side of said casing, a plurality of piezoelectric crystals embedded in an outer face of said block, individual leads for connecting said crystals in an electrical circuit, said leads extending through said block into said casing, a plurality of concentric cables having the center conductorsthereof connected to said leads, a conducting spacing rack for said concentric cables arranged Within said casing for connecting said casing and the outer conductors of said cables, a conduit connection for leading said cables from said casing, and a removable cover for the other of said sides of said casing for sealing said casing and for affording access to saidv rack.

8. In an underwater sound receiving system of the type employing a curved surface reflector, an electroacoustic transducer for receiving signals from such reflector and comprising a support, ablock of resilient insulating material mounted on said support and having a row of recesses along one face thereof, piezoelectric crystals embedded in said recesses and having exposed faces lying approxi- 7- 3 mately, along-the: line of fOCUSfROilJtSffOI. sig nalsreflected. 1,591,068 Williams July 6, 1926 onto saidiransducer, and eleetrzodemeansengaging; said 2,384,465 Harrison Sept. 11, 1945 crystals for conducting individual signals from said FOREIGN PATENTS 5 301,034 Great Britain Feb. 21, 1930 crystals.

References Cited in the file ofjthispatent UNITED STATES PATENTS 1,578,439 Walker Mar. 30, 1926 

